1. Field of the Invention
The present invention relates to a method of efficiently correcting die model data that have been generated on a CAD system into more accurate die model data.
2. Description of the Related Art
It has heretofore been customary to produce a press die by designing a die from the shape data of a formed article using a CAD system or the like to generate die data, then creating a numerical control (NC) program for machining a press die based on the die data, and machining a press die in a first stage on a numerically controlled (NC) machine tool which is operated by running the NC program. Since the machined press die in the first stage may not necessarily be able to produce formed articles of desired quality, it has been the general practice to check the press die based on formed articles that have actually be produced by the press die on a trial basis and correct the press die according to the results of the check.
For example, it has been proposed in the art to automatically correct die data produced using a CAD system by comparing the die data and measured dimensions of a formed article with each other, determining dimensional discrepancies caused by springback, shrinkage, etc., and correcting measured die dimensions using the dimensional discrepancies (see, for example, Japanese Laid-Open Patent Publication No. 2005-199567).
In order to make subsequent die corrections unnecessary, it has also been proposed to generate die model data, fabricate a full-size die model of synthetic resin according to the die model data, correct the die model, and then correct the die model data, after which an actual die is manufactured based on the corrected die model data (see, for example, Japanese Laid-Open Patent Publication No. 04-213704).
There has further been proposed a method of recognizing the deviation of the formed surface of a formed article from the forming surface of a die and the direction in which the deviation occurs, by measuring three-dimensional shapes and coordinate positions of the formed surface of the formed article placed on the die and article targets secured to the formed article, and comparing the three-dimensional shape of the die and the three-dimensional shape of the formed article with each other using the coordinate positions of die targets secured to the die and the coordinate positions of the article targets secured to the formed article (see, for example, Japanese Laid-Open Patent Publication No. 2006-234473).
Dies, such as upper and lower dies, for pressing articles of complex shapes, such as automobiles, tend to develop clearances between the mating surfaces thereof which cannot be predicted from prototype articles and pressing simulations, and the prototype articles are liable to suffer wrinkles and cracks. Therefore, it is necessary to repeat a process of correcting the dies and producing prototype articles again.
Since a group of die measuring points are corrected and thereafter corrected die data are regenerated based on the corrected group of die measuring points, it takes a long period of time to produce die data. When a repetitive die (second die) is to be produced, the die data are used as feedback data to generate die model data for the repetitive die. Therefore, the repetitive die can be designed in a relatively short period of time. Repetitive dies are produced, for example, to manufacture doors for one side of automobiles which are symmetrical to doors for the other side of automobiles after the die for the doors for the other side of automobiles has been produced, and also to manufacture identical products at a plurality of production sites.
For further shortening the time required to produce repetitive dies, the three-dimensional shape of a corrected die may be measured and the produced three-dimensional data may be reflected in die model data for the repetitive dies.
However, it is not easy to reflect the three-dimensional data in the die model data for the repetitive dies. According to a method of generating a polygonal model from a group of die measuring points and generating surfaces based on the polygonal model, it would not be possible to obtain CAD data that keep surfaces neatly joined to each other, are faithful to the die measuring points, and represent smooth surfaces. Specifically, as die surfaces contain small marks caused by a numerically controlled (NC) machining process, CAD data representing smooth surfaces may not be produced if measured die dimensions are directly reflected in die model data.
If data representative of surfaces are simply compared to each other for correcting positional deviations, then corresponding points that are defined in order to correct an area having a small radius of curvature or an area having a small shape tend to be in twisted association with each other.